The present invention is directed generally to a wireless spread spectrum device, and, more particularly, to a system and method for acquiring a received signal in a wireless spread spectrum device.
Spread spectrum modulation, including code division multiple access (CDMA) modulation, is one of several techniques for allowing a large number of system users to share a communication system. Other multiple access techniques include time division multiple access (TDMA) and frequency division multiple access (FDMA) systems. There are also analog frequency modulation (FM) based wireless communication systems, such as the Advanced Mobile Phone System (AMPS). In addition, many wireless communication devices employ global position system (GPS) techniques. Some wireless communication systems are capable of operating using multiple techniques, such as CDMA and GPS, or on different frequency bands, such as cellular or Personal Communication Services (PCS) bands.
To simplify the discussion, the background of the invention will focus on CDMA modulation techniques in a wireless communication system. The principles discussed, however, are generally applicable to any spread-spectrum system. CDMA modulation techniques are disclosed in U.S. Pat. No. 4,901,307, issued on Feb. 13, 1990, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, which is assigned to the assignee of the present invention, and the disclosure of which is incorporated herein by reference. The above-referenced patent discloses the use of a phase-coherent and chip-synchronous chip sequence that is defined as a pilot chip sequence, or pilot signal. The pilot signal can be used to provide phase and time acquisition and tracking, and multi-path correction.
Methods for acquiring signals are disclosed in the above-referenced patent and in the following patents: (1) U.S. Pat. No. 5,781,543, issued on Jul. 14, 1998 and entitled xe2x80x9cPOWER-EFFICIENT ACQUISITION OF A CDMA PILOT SIGNALxe2x80x9d; and (2) U.S. Pat. No. 5,805,648, issued on Sep. 8, 1998 and entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEMxe2x80x9d, both of which are assigned to the assignee of the present invention and the disclosures thereof are incorporated herein by reference.
When the wireless device is first powered on, the device must acquire a signal from a remote location, such as a base station transceiver system (BTS). A wireless CDMA communication device will typically be receiving pilot signals from a plurality of BTSs. The wireless device will search for the signals from the BTSs and will establish a communication link with a selected BTS to permit the reception and transmission of data, such as audio signals, over the established communication link. The selection of a particular BTS and the actual communication between the wireless communication device and the selected BTS are well known in the art and need not be discussed in detail herein.
As discussed in the above-referenced patents, in a CDMA communications system each BTS broadcasts the same pseudo-noise (PN) code pilot signal, but with a different phase offset. The pilot signal can be considered as a rotating phasor of the form:
s(t)=Ae(xe2x88x92xcfx89i+xcfx86).
To acquire the signal, the wireless CDMA device must synchronize with the phase, xcfx86, and frequency, xcfx89, of the signal transmitted by a BTS. The object of a xe2x80x9csearcherxe2x80x9d process in the wireless device is to find the phase, xcfx86, of the received signal. The searcher uses an estimated frequency, xcfx89. If the estimated frequency is not sufficiently close to the frequency of the pilot, the received signal will not be acquired.
Conventional search modes sequentially conduct a search of all possible PN phase offsets using a set of hypothetical search parameters. The search parameters may be varied before the next search sequence, or search sweep. Search parameters may include a search-slice size, or window, a coherent integration length, a non-coherent integration length, a presumed frequency error and Walsh and Quasi-Orthogonal Function (QOF) parameters, and other search parameters. Search parameters are discussed in more detail in the above-referenced U.S. Pat. No. 5,805,648. Conventional search methods are capable of searching for only a single pilot channel during each search sweep. In addition, conventional search methods are capable of using only a single presumed frequency error during each search sweep.
Conventional search methods perform reasonably well with single channel pilot signals broadcast over a bandwidth of approximately 1.25 MHz, or 1xc3x97 signals. Conventional search methods also perform reasonably well when the frequency error range is low. Recently, several CDMA technologies, such as 3xc3x97 direct spread (3xc3x97DS) signals, 3xc3x97 multi-carrier (3xc3x97MC) signals and orthogonal transmit diversity (OTD) signals, have been developed that broadcast the pilot signal over broader frequency ranges or in multiple channels. In addition, the frequency error range in the newer CDMA technologies may be higher than in the 1xc3x97 signals. Conventional search methods fail to take advantage of newer CDMA broadcast technologies. Conventional search methods also have a limited frequency range over which the phase of the pilot signal will be detected.
Therefore, it can be appreciated that there is a significant need for an improved system and method for acquiring a signal in a wireless spread-spectrum device. The present invention provides this and other advantages, as will be apparent from the following detailed description and accompanying figures.
The present invention is embodied in a system and method for acquiring a signal in a wireless spread-spectrum device, which provides greater flexibility than prior art acquisition architectures. In one embodiment, the present invention may be configured to search multiple channels for the signal during each search sweep. In another embodiment, the present invention may be configured to use multiple presumed frequency errors during each search sweep. In an exemplary embodiment, the present invention may be configured to search multiple channels using multiple presumed frequency errors during each search sweep.